If you don't have plans to see this movie, you can check the spoilers here and then come back.
A tremendous waste of a pretty impressive cast. The gist is that Paul Rudd's Ned is a well-meaning simpleton who ends up homeless and thus works his way through each of his three sisters' homes and lives, unintentionally screwing up their relationships along the way, but ultimately helping them. If the premise seems a bit thin, that's because it is; it's a bad script with a predictable plot you can see coming at every turn. Despite this, the movie succeeds in spite of itself in some moments simply because Rudd makes bits that shouldn't be--and wouldn't be in the hands of another actor--funny by sheer force of will. However, considerable as it is, Paul Rudd's charm can only carry a film so far, and even he can't make up for a lame attempt to squeeze an hour and a half of missing character development into the final 15 minutes. The most frustrating part of Our Idiot Brother is, however, as mentioned, the wasted talent: Zooey Deschanel is miscast and barely used, Elizabeth Banks is forced into being the uptight sister when she works much better playing carefree or least wild, and Emily Mortimer spends more time attempting to mask her British accent than putting on a decent performance. Adam Scott shows flashes of potential as Banks' love interest, so he gets cut from most of the movie, Steve Coogan pushes the dial too far past loathsome as Mortimer's adulterous husband and Rashida Jones is uneven as Deschanel's lawyer girlfriend. The funniest and best-used supporting players are Kathryn Hahn as Rudd's passive agressive hippie ex, T.J. Miller as her clueless new stoner beau and Sterling K. Brown as an incredulous parole officer. I'd love to see this same cast in a better movie.